a. Field of the Invention
The present invention relates to a method for the purification of raw caprolactam containing, as impurities, one or more primary amides having the formula I ##STR2## wherein R is a hydrocarbon radical having from 1 to 14 carbon atoms, in particular hexahydrobenzamide.
b. Prior Art
Methods for purifying caprolactam from the amides which it contains are known. Said methods may be grouped in two different classes, chemical and physical methods. Thus, e.g., a chemical method is claimed in the Japanese patent application No. 46-23751, and is characterized by the fact that raw caprolactam is treated with a solution of sodium hypochlorite. Another chemical method is claimed in DOS No. 1.926.932 and consists in treating the molten raw caprolactam with metal oxides. The physical methods mainly consist in distilling and/or rectifying the caprolactam. Thus e.g. in English Pat. No. 1,157,416 a method is described for purifying caprolactam by distillation. In Italian patent application No. 20612 A/79 of the Applicant a method is described for purifying raw caprolactam by combination of distillation and rectification operations.
The aforesaid hitherto known chemical and physical methods for the purification of caprolactam have, however, considerable disadvantages. Thus e.g., the chemical purification methods have the disadvantage that they are not selective, inasmuch as besides the amides of formula I, the caprolactam too may react for example with the hypochlorite, whereby new products such as chlorolactam are formed, from which the caprolactam can be separated and isolated only with difficulty.
The disadvantages of the known physical methods may be summed up in the fact that it is difficult, in the purification of caprolactam by rectification, quantitatively to separate two products having boiling points close to one another, unless a rectification column having a very high number of plates is used.
In general there is a tendency to collect or separate one of the two products in the purest possible form to leave a part of the other product mixed with the first and to discharge the mixture as heads or tails of the distillation. When, as in the case of caprolactam, the compound to be purified is sensitive to heat, the use of a very large number of plates or high reflux ratios may cause the distruction of a part of the product or secondary reactions such as polymerization, which is very harmful because it causes the loss of yield and makes the operation of the apparatus difficult.
In the purification of caprolactam by rectification, as disclosed in Italian patent application No. 20612 A/79, the highest boiling amides of formula I, in particular hexahydrobenzamide, concentrate in the distillation tails, always however together with part of the caprolactam. The resulting residue therefore always contains a large amount of caprolactam which must be subsequently recovered to render the processes economical.
Italian patent application No. 20612 A/79 contains the following disclosure:
"This invention relates to a method of purifying caprolactame, e.g. caprolactame of the same grade as yielded by reacting nitrosyl compounds with carbocyclic derivatives.
It is a well known fact that the aforesaid reaction yields, additionally to caprolactame, small amounts of non-cyclic amides, such as acetamide, popionamide, butyramide, n-valeramide, benzamide, tetrahydrobenzamide, hexahydrobenzamide, and morever, still in small amounts, aliphatic and aromatic cyclocarboxylic acids, epsilon-aminocapronic acid, and unsaturated oxidable substances.
Several methods of purifying caprolactame are known in the art. Thus, for example, when effectuated by reacting nitrosyl compounds with carbocyclic derivatives, the purification of caprolactame (following its extraction from the sulphuric reaction mass and known steps for separating raw caprolactame) is substantially carried out, according to current practice, by means of two separate and discrete chemical treatments, a first one directed to destroying the amides, and a second one directed to reducing or eliminating the easily oxidable substances. More specifically, these treatments consist of subjecting firstly the raw caprolactame to the action of sodium hypochlorite (conversion of the amides into more volatile amines), and subsequently to an oxidizing action, e.g. with potassium permanganate or ozone (decomposition of the oxidable substances); the product, after undergoing that treatment, is subjected to distillations, possibly under vacuum conditions, without rectification, thereby a "polymerization grade" caprolactame is obtained. Said chemical treatments, however, have the following drawbacks:
(1) they are expensive; PA1 (2) they yield impurities of a different nature (such as chloro-lactames, for instance) which, although only some ppm, result in contamination of the caprolactame; PA1 (3) the treatments with hypochlorite-permanganate, respectively hypochlorite-ozone, are not absolutely selective, even when the very high caprolactame-byproducts ratio is taken into account, thereby part of the caprolactame is destroyed by these chemicals, with an attendant appreciable decrease of the yield. PA1 Step (a)--100.degree. to 150.degree. C. PA1 Step (b)--vapors, 110.degree. to 150.degree. C.; reboiler, 120.degree. to 180.degree. C.; PA1 Step (c)--100.degree. to 150.degree. C. PA1 Temperatures in the 125.degree. to 130.degree. C. range; PA1 Residual pressure, 4 mm Hg; and for the rectification of step (b): PA1 Head temperatures in the 105.degree. to 110.degree. C. range; PA1 Reboiler temperature in the 120.degree. to 165.degree. C. range. PA1 Volatile bases, 0.1 meq/kg; PA1 Permanganate Number, 15,000 seconds; PA1 HAZEN color, less than 5; PA1 Absorption at 290 nm, 0.008; PA1 Absorption summation from 260 to 300 nm, 0.290.
The applicant has now surprisingly found a method of purifying caprolactame by the application of only one line of physical purification. With this process it becomes possible to obtain a caprolactame of a purity even higher than that obtainable with the use of hypochlorite-permanganate, respectively hypochlorite-ozone.
By not applying said treatments of a chemical nature, not only all of the aforesaid disadvantages are eliminated, but a lower cost and improved environmental protection can also be achieved in the purification process.
This invention sets out to provide a caprolactame purification process by vacuum distillation, characterized in that said distillation is carried out, in a continuous and/or discontinuous (batch distillation) manner, in the following stages or steps, in the same order as listed herebelow:
(a) fast vacuum distillation of the raw caprolactame, possibly in the presence of an alkaline and/or alkaline-earth hydroxide; PA0 (b) distillation with vacuum rectification of the caprolactame yielded as distillate from (a), with separation from the high-boiling, and possibly low-boiling, byproducts; and PA0 (c) fast vacuum distillation of the caprolactame yielded from (b), in the presence of an alkaline and/or alkaline-earth hydroxide. PA0 From Step (a), a distilled portion corresponding to 90-98% by weight with respect to the raw caprolactame, and 2-10% by weight of residue, again with respect to the raw caprolactame; PA0 From Step (b), a head fraction, corresponding to 2-5% by weight with respect to the caprolactame supplied from Step (a), a core fraction, corresponding approximately to 90% by weight of the caprolactame supplied from Step (a), and a residue corresponding to 5-8% by weight again with respect to the caprolactame supplied from Step (a); PA0 From Step (c), a fast distillate to yield a residue corresponding to approximately 5-10% by weight of the caprolactame supplied from Step (b).
The term "fast distillation", as used herein, is intended to describe an operation wherein the evaporated material is no more recycled, in any substantial manner, into the evaporation zone. The percentage by weight of the alkaline or alkaline-earth hydroxide, at step (c), and possibly (a) as well, with respect to caprolactame, varies preferably in the 0.05% to 5% range, and more preferably from 0.1% to 1.0%.
Advantageously, the temperature of the various steps (a), (b) and (c) are preferably maintained within the following limits:
As the alkaline hydroxide, potassium hydroxide is used of preference, and more preferably sodium hydroxide, whereas when an alkaline-earth hydroxide is used, calcium hydroxide is preferred; however, as the hydroxide, according to this invention, sodium hydroxide is still more preferably employed. Said alkaline or alkaline-earth hydroxides are preferably employed, in accordance with this invention, in the form of a solution, and more preferably of an acqueous solution.
According to a variant of the invention, (mainly suggested by economics and physical-mechanical considerations) the purification of caprolactame is carried out at the steps (a), (b) and (c) such as to obtain:
In step (b), the head fraction may or may not be separated from the caprolactame which is supplied to the successive step (c).
The operating conditions which can be adopted for the two fast distillations, as per steps (a) and (c), are the following:
The finally yielded caprolactame has the following characteristics:
None of the individual operations, when carried out separately and/or in a different order from the one provided by the invention, yields caprolactame having the aforesaid characteristics.
A further object of this invention includes the purified caprolactame as obtained with the purification method described hereinabove.
The following example should be considered as merely illustrative and in no way limitative of this invention.